The invention relates to a doppler radar apparatus, comprising transmitter means and receiver means coupled to antenna means, and processor means coupled to the receiver means, for the detection of helicopters by blade flashes caused by helicopter rotor blades and the identification of helicopters by determining the repetition frequencies of the blade flashes.
Such a radar apparatus is known from the Patent Specification U.S. Pat. No. 4,389,647. In the above-mentioned radar apparatus, output signals of a Doppler radar are rectified and passed through a high-pass filter. In the event of a helicopter the resulting blade flashes, i.e. strong radar echoes caused by the helicopter rotor blades which are briefly perpendicular to the radar beam, will produce a signal at the output of the high-pass filter. The repetition frequency of this signal, which corresponds with the repetition frequency of the blade flashes makes it possible to draw a conclusion as to the helicopter type.
This method has the drawback that valuable information concerning the type of helicopter is lost in the course of the rectifying process.